1. Field of the Invention
The present invention relates to a semiconductor laser device which is preferably used as a light source of a magneto-optical pick-up device for recording and/or reproducing information onto or from an information recording medium.
2. Description of the Related Art
A conventional semiconductor laser device used for magneto-optical pick-up (Japanese Laid-open Publication No. 6-168462) will now be described with respect to its configuration and operation.
First, the configuration of the conventional semiconductor laser device will be described. FIG. 30 is a diagram showing an optical system of the conventional semiconductor laser device and an information recording medium. Referring to FIG. 30, a semiconductor laser element 101 and a servo-signal light-receiving element 102 for detecting a focus error signal and a radial error signal are provided within a semiconductor laser unit 107. A polarization beam splitter 111, a collimator lens 112 and an objective lens 113 are sequentially placed in this order in the optical path from the semiconductor laser element 101 to an information recording medium 114. The polarization beam splitter 111 is secured on the top of the semiconductor laser unit 107. A diffraction grating 109 is formed at the surface of the polarization beam splitter 111 which faces the semiconductor laser element 101. Moreover, an information-signal light-receiving element 104 is provided outside the optical path between the semiconductor laser element 101 and the information recording medium 114. The information-signal light-receiving element 104 is divided into two elements for p-polarized light components and s-polarized light components, respectively. A Wollaston prism 116 is provided at the surface of the polarization beam splitter 111 which faces the information-signal light-receiving element 104.
Next, the operation of the conventional semiconductor laser device will be described. Light is emitted from the semiconductor laser element 101 onto the information recording medium 114. The light reflected from the information recording medium 114 (hereinafter, the reflected light is referred to as return light) passes through the objective lens 113 and the collimator lens 112 into the polarization beam splitter 111. The polarization beam splitter 111 partially reflects the return light into the Wollaston prism 116, while transmitting the remaining return light therethrough. The Wollaston prism 116 has different refractive indices for p-polarized light and s-polarized light, respectively. Therefore, the return light entering the Wollaston prism 116 is divided into a p-polarized light component and an s-polarized light component in the Wollaston prism 116. The two elements of the information signal light-receiving element 104 are located at the positions on which the two divided light components for an information signal are focused, respectively. The information signal is calculated based on an output of the information-signal light-receiving element 104. The remaining return light having passed through the polarization beam splitter 111 is diffracted by the diffraction grating 109 into the servo-signal light-receiving element 102. The focus error signal and the radial error signal are detected based on an output value from the servo-signal light-receiving element 102.
According to the conventional semiconductor laser device shown in FIG. 30, the information-signal light-receiving element 104 is separately provided outside the semiconductor laser unit 107, whereby the overall size of the device is increased.
In order to provide a smaller and thinner semiconductor laser device which solves the above-mentioned problem, an information-signal light-receiving element may be placed within a semiconductor laser unit, as shown in FIG. 31. Such a semiconductor laser device will now be described with respect to its configuration and operation with reference to FIG. 31.
FIG. 31 shows another conventional semiconductor laser device and an information recording medium. First, the configuration of this conventional semiconductor laser device will be described. Referring to FIG. 31, a semiconductor laser element 201 and servo-signal light-receiving elements 202 and 203 are provided within a package 205. The package 205 is sealed by a transparent seal substrate 206. Thus, a semiconductor laser unit 207 is configured. A light-transmitting substrate 208, a collimator lens 212 and an objective lens 213 are sequentially provided in this order in the optical path from the semiconductor laser element 201 to an information recording medium 214. A hologram optical element 228 includes a diffraction grating 209 and a three-beam generating diffraction grating 210. The diffraction grating 209 is formed at the surface of the light-transmitting substrate 208 which faces the collimator lens 212, whereas the three-beam generating diffraction grating 210 is formed at the surface of the light-transmitting substrate 208 which faces the seal substrate 206.
Hereinafter, the operation of the conventional semiconductor laser device shown in FIG. 31 will be described. Light emitted from the semiconductor laser element 201 is divided into three light beams by the three-beam generating diffraction grating 210. More specifically, the three-beam generating diffraction grating 210 divides incident light into positive first-order light which is diffracted in the direction perpendicular to the plane of FIG. 31 from the rear to the front of the plane of FIG. 31, 0th-order light which is not diffracted, and negative first-order light which is diffracted in the direction perpendicular to the plane of FIG. 31 from the front to the rear of the plane of FIG. 31. The three light beams thus divided pass through the hologram optical element 228, and then, through the collimator lens 212 and the objective lens 213 so as to be focused onto the information recording medium 214. The light beam reflected from the information recording medium 214, that is return light, is directed back to the hologram optical element 228 through the same optical path. Thereafter, the return light is diffracted by the diffraction grating 209 of the hologram optical element 228 so as to be focused onto a focus-error-signal light-receiving region (not shown) and a radial-error-signal light-receiving region (not shown) of the servo-signal light-receiving elements 202 and 203. Each of the focus-error-signal light-receiving region and the radial-error-signal light-receiving region is divided into a plurality of elements. A focus error signal is detected by first converting a current output from each element of the focus-error-signal light-receiving region to a voltage, and then, performing a differential operation of the voltages thus converted. A radial error signal is similarly detected by a differential detection method using a three-beam method. An information signal is obtained by first converting a current output from each element of the focus-error-signal light-receiving region to a voltage and then calculating the sum of the voltages thus converted.
The conventional semiconductor laser device shown in FIG. 31 obtains the information signal by calculating the sum of the signals from the plurality of elements. Therefore, a noise component of the signal from each element is added. As a result, the total noise component is increased according to the number of elements, causing significant reduction in a signal/noise (S/N) ratio.